Acoustic measurements using currently available tools and methods provide key information in oilfield borehole logging. The currently available acoustic tools are useful in providing a wide range of information regarding the surrounding formation and the borehole parameters. Surface seismic and vertical seismic profiling (VSP) methods are utilized to provide imaging of the overall geological structure of a hydrocarbon reservoir. Sonic and other well logging methods provide good resolution imaging in the immediate vicinity of the borehole. Sonic imaging is yet another technique that bridges the gap in spatial resolution between these seismic and well logging methods.
Some tools include a single source of sonic waves and two or more receivers, however, most of the tools now include two or more acoustic sources and many receivers arranged in an array. A primary use of acoustic borehole measurements is the estimation of compressional (P) wave and/or shear (S) wave formation slowness. The estimation of compressional and/or shear wave formation slowness is often expressed as an S-T (slowness vs. time) plane, and can be visualized at the wellsite with current technology.
While sonic imaging generally has been successful, waveform data acquired for sonic imaging purposes typically contain many types of arrivals in addition to the desired reflected arrivals, such as tool-borne noise and borehole-borne noise. For example, direct compressional and shear headwaves and tube waves, which are a part of the raw data that are acquired during downhole acoustic measurements, tend to obscure the reflected arrival waves relating to acoustic reflectors in subterranean formations.
The assignee of the present application, Schlumberger, has developed a wireline sonic imaging tool (referred to as the Borehole Acoustic Reflection Survey (BARS) tool) that allows reservoir features such as reflectors and fractures to be imaged.
Borehole acoustic reflection surveys are performed, for example, for the characterization of local geology, validation of geo-steering, and detection of fractures. The acquired sonic waveforms are processed and migrated to obtain images up to about 10 to 20 meters away from the borehole. In this, it is desirable to have improved techniques for modeling the dominant wavefields, such as the direct compressional, shear and Stoneley waves, for example, so that they may be removed, making subsurface imaging and characterization efficient and reliable.
Typically, in sonic data processing the reflected (p-to-p, p-to-s and s-to-p) and refracted (p-to-s and s-to-p) waves are usually weak compared to the direct compressional, shear and Stoneley waves. One important aspect of sonic data processing involves the efficient separation of wavefields to reliably extract the reflected and refracted signals. Known techniques for wavefield separation include the adaptive interference canceller (AIC) filter and median velocity filters in common offset gathers, i.e., gathers of traces recorded by common receiver stations of the sonic array tool. However, in typical sonic data processing to extract the reflected and refracted signals event signals whose apparent velocity is slower than the direct P-waves may be removed and the amplitudes, especially for events from layers which are parallel to the well, tend to become weaker. Therefore, it is also desirable to have techniques for modeling the dominant wavefields with greater precision so that extraction of the reflected and refracted wavefields conserves amplitudes of event signals.
The limitations of conventional data processing noted in the preceding are not intended to be exhaustive but rather are among many which may reduce the effectiveness of previously known techniques. The above should be sufficient, however, to demonstrate that acoustic data processing techniques existing in the past will admit to worthwhile improvement.